1. Field of the Invention
This invention pertains to the field of electronic security systems that detect unwanted intrusions into secured areas and sound an audible alarm in response thereto. More particularly, the invention pertains to a method of differentiating between a high intensity shock or substantial threat as well as a low intensity or insubstantial threat, received by the protected structure, and sounding an appropriate alarm as well as preventing non-shock energy inputs from tripping the security alarm.
2. Description of the Prior Art
Electronic security systems have been used for some years and their popularity increases as the national crime rate continues to climb. Most such systems, especially those used in structures such as automobiles, include a controller, a series of intrusion sensors such as triggers for detecting attempted intrusions through doors, hood, and windows, an alarm for activation upon receipt of a signal or signals from the sensors indicating an attempted unwanted entry into the vehicle, and a power source, normally the vehicle battery, to power the system and sound the alarm. Other components are often included such as automatic resetting circuits and shut-down devices for use when the alarm needs to be deactivated. These systems may be original equipment on new vehicles or retro-fitted on existing vehicles.
The security systems may be activated and deactivated by an RF signal, usually a digitally coded signal specific to the particular vehicle, that is sent by a hand-held transmitter operated by one or more push buttons activated by the car owner. In addition, other such systems may be activated by mere passage of time following the driver's act of turning off the motor and exiting the vehicle with the doors and windows closed and after a short time interval such as 30 seconds. Thereafter the system may be deactivated by a hand-held transmitter or by a delay circuit that activates the alarm if the system is not deactivated by the driver upon entry into the vehicle. The first type of activation is known as "active arming" while the latter is known as "passive arming".
Upon detection of an attempted intrusion into the vehicle, from either an input from a trigger or a sensor, the alarm is activated for a period of time, for instances 45 seconds to one minute, and then, if the alarm has not been deactivated by the remote transmitter or by the manipulation of a "kill" switch mounted in the interior of the vehicle, usually in a hidden area therein, the alarm is deactivated and the security system is once again reset to monitor the sensors and triggers.
One form of such a sensor is called a "shock" sensor and could comprise a permanent magnet suspended in an elastic mount such as between one or more rubber bands adjacent to an induction coil wired into an electronic security circuit interior to the vehicle. An incoming mechanical shock to the vehicle, such as an impact from another automobile, or a person lightly opening a car door against the vehicle, or a person tapping the car window with a coin, causes the magnet to vibrate in the rubber bands causing the magnetic field surrounding the magnet to interact with the induction coil and induce an alternating electric voltage in the coil. The voltage passes from the coil to an amplifier and if it exceeds a base reference value impressed on the amplifier circuit, is passed on through to the alarm whereupon the alarm is activated.
Certain problems exist with these security systems that render their usage less than desirable under certain circumstances. For instance, a shopping cart inadvertently lightly bumped against the vehicle will usually cause the full alarm to go off. While the alarm is certainly necessary to alert the owner, inadvertent tripping of the alarm is annoying and could result in either the owner becoming frustrated and thereafter not activating the alarm in a crowded parking lot or convincing the shopper or other car owner that such a loud, annoying alarm is not what he or she wants in their particular car.
In other situations, certain transient electric fields can generate enough of a signal in the induction coil to trip the alarm even in the absence of a physical touching of the vehicle. In the case where the owner turns on the headlights for night driving, the surge of direct current to the headlights and running lights can generate such an EMF pulse that its interaction with the sensor induction coil will generate a strong enough signal to be picked up by the induction coil, amplified and trip the alarm. Other situations, such as where the owner uses a cellular telephone in the vehicle, the initial surge of RF energy from the cellular telephone in the vehicle to link up with the telephone circuit is often sufficient to generate an actuation level signal. Still further, in isolated cases, such as where a police car parks behind a motorist and the officer "keys" the microphone on his radio, the RF surge from his transmitter could interact with the anti-theft system induction coil and produce a false alarm.
Still further, there are instances where a disturbance continues unabated after the initial activation of the alarm sequence. For instance, a vehicle parked next to a train station may receive an input as a train passes. The alarm will commence and terminate yet the train has not passed completely by the vehicle. In the prior art, the alarm will sound again because of the continuous input of energy from the train. This can be of annoyance to others in the area.
Crowded parking lots are prime areas for car theft. In these cases, dissatisfaction with the anti-theft system may cause the owner to cease arming the system thus leaning the vehicle unprotected. This condition, if not corrected, may cause other vehicle owners to cease purchasing any such security system for fear of annoying others.
What is needed to deal with these circumstances is (1) a vehicle security sensor that will differentiate between the strength of a light, generally non-threatening shock and a stronger, usually security-threatening shock to the vehicle and output a pulse to the alarm circuit appropriate to the strength of the incoming shock; and, (2) a vehicle security system that will look for these unwanted non-physical threatening shocks and block them or otherwise divert the signals they produce so that an alarm is not generated.